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THE INFLUENCE OF MARINE MICROFOULING ON THE CORROSIONBEHAVIOUR OF PASSIVE MATERIALS AND COPPER ALLOYS

Brenda J. Littlea, Jason S. Lee b and Richard I. Rayc

aNaval Research Laboratory, Code 7303, 228-688-5494, blittle@nrlssc.navy.milbNaval Research Laboratory, Code 7332, 228-688-4494, ilee@nrlssc.navy.miNaval Research Laboratory, Code 7332, 228-688-4690, ray@nrlssc.navy.mil

ABSTRACT

The influence of marine biofilms on corrosion varies with alloy composition. At temperaturesbelow 600 C, resistance to crevice corrosion is the limiting factor for selecting alloys for seawaterservice and is the most problematic issue affecting the performance of passive alloys in seawater.Several investigators have documented the tendency for biofilms to cause a noble shift, or anennoblement, in open-circuit potential (OCP) of passive alloys exposed in marine environments.Ennoblement in marine waters has been ascribed to depolarization of the oxygen reductionreaction due to organometallic catalysis, acidification of the electrode surface, the combinedeffects of elevated H20 2 and decreased pH and the production of passivating siderophores. Thealloys tested include, but are not limited to: UNS S30400, S30403, S31600, S31603, S31703,S31803, N08904, N08367, S44660, S20910, S44735, N10276, N06625, platinum, gold,palladium, chromium, titanium, and nickel. Theoretically, potential ennoblement should increasethe probability for pitting and crevice corrosion initiation and propagation, especially for alloyswith pitting potentials within 300 mV of the OCP. The relationship between passive alloycomposition and ennoblement will be discussed.The well-known toxicity of cuprous ions toward living organisms does not mean that the copper-based alloys are immune to microbial colonization and microbiologically influenced corrosion(MIC). It does mean, however, that only those organisms with a high tolerance for copper arelikely to have a substantial effect. Most of the reported cases of MIC of copper alloys in marineenvironments are not related to ennoblement of OCP, but are caused by the reduction of sulfate(concentration > 2 gm L-) to hydrogen sulfide. Sulfate-reducing bacteria (SRB) produce sulfide-rich conditions in the biofilm and the oxide layer on the metal (or the metal itself) is destabilizedand acts as a source of metal ions. At the outer surface of the SRB these ions react to producesulfide compounds in micron-sized particles that are in some cases crystalline. The consumptionof metal ions at the microbe surface is balanced by release of surface ions until the oxide is totallyconsumed. Susceptibility of copper alloys to derivatization by microbiologically producedsulfides depends on alloying elements. However, in the presence of turbulence, the looselyadherent sulfide film is removed, exposing a fresh copper surface to react with the sulfide ions.For these reasons turbulence-induced corrosion and sulfide attack of copper alloys cannot bedecoupled easily. In the presence of oxygen, the possible corrosion reactions in a copper sulfidesystem are extremely complex because of the large number of stable copper sulfides, theirdiffering electrical conductivities, and catalytic effects. Transformations between sulfides, or ofsulfides to oxides, result in changes in volume resulting in internal stresses that weaken theattachment scale and oxide subscale leading to spalling. In contrast, many passive alloys areimmune to sulfide-induced corrosion.

Keywords: Marine biofilms, passive allovs. conner allov; and ennnhlf-mPnt

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02-01-2008 Conference Proceeding

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The Influence of Marine Microfouling on the Corrosion Behaviour of PassiveMaterials and Copper Alloys 5b. GRANT NUMBER

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6. AUTHOR(S) 5d. PROJECT NUMBER

Brenda J. Little, Jason S. Lee, Richard I. Ray

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Naval Research Laboratory REPORT NUMBER

Oceanography Division NRL/PP/7303-07-7139

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The influence of marine biofilms on corrosion varies with alloy composition. At temperatures below 60'C, resistance to

crevice corrosion is the limiting factor for selecting alloys for seawater service and is the most problematic issue

affecting the performance of passive alloys in seawater. Several investigators have documented the tendency for

biotilms to cause a noble shift, or an ennoblement, in open-circuit potential (OCP) of passive alloys exposed in marine

environments. Ennoblement in marine waters has been ascribed to depolarization of the oxygen reduction reaction due

to organometallic catalysis, acidification of the electrode surface, the combined effects of elevated 11202 and ...

15. SUBJECT TERMS

marine biofilms, passive alloys, copper alloys, and ennoblement

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a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Brenda J. LittlePAGES

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Standard Form 298 (Rev. 8/98)Prescribed by ANSI Std. Z39.18

INTRODUCTIONExposure of any engineering material toa naturally occurring seawater initiates a series ofsequential and parallel biological and chemical events that culminate in the formation of a complexlayer of inorganic, organic, and cellular components known as biofouling. Biofouling is a generalterm used to describe both the microbiological and macrobiological growths that develop onexposed surfaces. Gubner and Beech (1999) evaluated the parameters that affect the corrosivity ofnatural seawater and concluded that the general microbial population, in addition to specific typesof marine microorganisms, i.e., sulfate-reducing (SRB), sulfur-oxidizing bacteria (SOB) andchemo-organotrophic bacteria, could affect corrosivity. The influence of micro- and macrofoulingon corrosion rate can range from acceleration to inhibition.

DISCUSSIONMicrofouling

Natural seawater surrogates, including sodium chloride (NaCI) solutions (3.5 wtO/o), buffered NaCIsolutions, artificial seawater and artificial seawater to which nutrients and microorganisms have been added,have been used in microbiologically influenced corrosion (MIC) experiments. It is generally recognized thatartificial seawater mixtures do not approximate the complexity of natural seawater, especially the organicmaterial and the microflora. The assumption has been that NaCI solutions were free of organics andmicroflora. Dexter (1988) concluded that NaCI solutions were not free of organics. Instead, the organics inNaCI solutions were just different from those found in natural seawater. Over time, NaC! mixtures becomecontaminated with bacteria, fungi and microalgae. Yet the presence and activities of these organisms areusually not quantified or considered in corrosion experiments. Most corrosion experiments conducted inartificial seawater do not reproduce rates and in some cases the mechanism for corrosion in natural seawater(LaQue, 1975).

Zobell and Anderson (1936) and Lloyd (1937) demonstrated that when seawater was stored in glassbottles, the bacterial numbers fell within the first few hours followed by an increase in the total bacterialpopulation with a concurrent reduction in the number of species. Lee et al. (2007) demonstrated dramaticchanges in the chemistries and microflora of two natural coastal seawaters as a result of storage andenvironmental conditions (Figure 1). Exposure to an anaerobic atmosphere containing a mixed gas ofnitrogen (N2), carbon dioxide (CO2), and hydrogen (H2) generated the highest microflora concentration,especially SRB. Biotic dissolved sulfide levels were also highest in the mixed gas atmosphere. In contrast,sulfides were not detected in anaerobic seawater maintained with bubbled N2. The pH of natural seawater iscontrolled by CO2. The concentration of dissolved gases (02 and C02) in surface waters is determined bytheir solubilities, which are functions of salinity and temperature (Dexter, 1998). As more CO2 is dissolvedthe pH decreases. If CO2 is removed from solution (by N2 purging), pH increases. Lee et al. (2007)demonstrated that bubbling N2 into natural seawater produced a pH shift from 8.0 to above 9.0, anenvironment that was not conducive to the growth of SRB. Maintenance of seawater in an anaerobic hoodwith an anaerobic mixture of gases produced a pH shift of 8.0 to below 7.0 and a significant increase in SRBnumbers.

Natural seawater and passive alloys

At temperatures below 60'C resistance to crevice corrosion is the limiting factor for selecting alloys forseawater service and crevice corrosion is the most problematic issue affecting the performance of stainlesssteels in seawater. Several investigators (Chandrasekaran and Dexter, 1994; Johnsen and Bardal, 1985;Motoda et al. 1990; Ito et al., 2002; Zhang and Dexter., 1995) have documented the tendency for marinebiofilms to cause a noble shift, or an ennoblement, in open circuit potential (OCP) of passive alloys. Thealloys tested include, but are not limited to: UNS S30400, S30403, S31600, S31603, S31703, S31803,N08904, N08367, S44660, S20910, S44735, N10276, N06625, platinum, gold, palladium, chromium,titanium, and nickel (Figure 2). Ennoblement of OCP has been reported in fresh, brackish and seawaters. Infresh and brackish water, ennoblement is related to microbial deposition of manganese (Dickinson et al.,

1996). Ennoblement in marine waters has been attributed to depolarization of the oxygen reduction reactiondue to organometallic catalysis, acidification of the electrode surface, the combined effects of elevatedhydrogen peroxide (H202) and decreased pH and the production of passivating siderophores. Despite theextensive literature on the subject, the exact mechanism of ennoblement of metals in seawater remainsunresolved.

Theoretically, OCP ennoblement should increase the probability for pitting and crevicecorrosion initiation and propagation. However, attempts to relate biofilms to increased localizedcorrosion have been inconsistent. Part of the confusion about the significance of ennoblement canbe attributed to differences between exposure sites and exposure techniques. Ennoblement has beenmeasured for metals boldly exposed and metals incorporated in crevice assemblies. In some cases,anodes have been initiated galvanostatically and cathodes have been pre-ennobled with biofilms.Martin et al. (2007) compared ennoblement at two coastal seawater locations - Key West, Floridaand Delaware Bay (Figures 3a&b). The two locations have different temperatures and differentsalinities. Martin et al. (2007) demonstrated that OCP ennoblement is site specific, varying 100 mVvs. saturated calomel electrode between locations, with higher potentials at Delaware Bay.Localized corrosion was observed for alloy SS3040 boldly exposed in Key West (Figure 4), but notin Delaware Bay. Zhang and Dexter (1995) used a remote crevice assembly technique to expose aseries of metals in Delaware Bay. They initiated polarization in one set of specimens and comparedthe results with a set that was exposed without prior polarization. Biofilms did not affect initiationtimes for S31603 and S31725. Biofilms did increase the propagation rate for preinitiated UNSS31603, S31725 and N08904 as measured by maximum and average pit depths, weight loss andcurrent density.

Natural Seawater and non-passive alloys

Most of the reported cases of MIC of non-passive alloys (e.g., carbon steel and copper alloys) in marineenvironments are not related to ennoblement of OCP, but are caused by the reduction of sulfate(concentration > 2 gm L1) to sulfide by SRB. McNeil and Odom (1994) developed a thermodynamic modelfor predicting SRB influenced corrosion based on the likelihood that a metal would react withmicrobiologically produced sulfide. The model is based on the assumption that SRB MIC is initiated bysulfide-rich reducing conditions in the biofilm and that under those conditions, the oxide layer on the metal(or the metal itself) is destabilized and acts as a source of metal ions. At the outer surface of the SRB theseions react to produce sulfide compounds in micron-sized particles that are in some cases crystalline. Theconsumption of metal ions at the microbe surface is balanced by release of surface ions until the oxide istotally consumed. If the reaction to convert the metal oxide to a metal sulfide has a positive Gibbs freeenergy under surface conditions, the sulfides will not strip the protective oxide and no corrosion will takeplace. If the Gibbs free energy for that reaction is negative, the reaction will proceed, sulfide microcrystalswill redissolve and reprecipitate as larger, generally more sulfur-rich crystals, ultimately altering the sulfideminerals stable under biofilm conditions. In the presence of dissolved oxygen, the sulfides may react to formother compounds. Based on their model they prepared the following database:

* Ag - acanthite (Ag 2 S).* Ag-Cu alloys - acanthite, argentite (the high temperature polymorph of Ag 2S orjalpaite

(Ag 3CuS 2)).• Cu - complex suites of sulfide minerals: the most common product is chalcocite (Cu2 S).

Final product in many cases is blue-remaining covellite (CuSJ+x)." Cu-Ni alloys - Sulfide corrosion products similar to those of Cu but with significant djurleite

(Cu 31 S 16). No Ni minerals observed.* Cu-Sn alloys - Corrosion products similar to those in Cu.* Fe (carbon steel) - Final product is pyrite (FeS) with numerous intermediates.

" Fe (stainless alloys) - Rates are slower than pure Fe or carbon steel. No Ni minerals havebeen detected. Stainless steels with 6% or more Mo appear to be very resistant.

• Ni - millerite (NiS)." Pb - galena (PbS).* Sn- No data.* Ti - Not affected by SRB." Zn - SRB MIC produces a sulfide conjectured to be sphalerite, ZnS.* Zr - Not affected by SRB.

The model accurately predicts that titanium will be immune to reactions of sulfide along with most stainlesssteels and that carbon steel and copper alloys will be vulnerable to sulfide derivatization. The model islimited to thermodynamic predictions as to whether or not a reaction will take place and does not considermetal toxicity to the organisms, tenacity of the resulting sulfide or others factors that influence corrosion rate.

The severity of the localized corrosion resulting from sulfide derivatization depends on the presenceof oxygen. Hamilton (2003) proposed a model for SRB corrosion of carbon steel in which sulfate, anintermediate electron acceptor, is reduced to sulfide. In his model, sulfide reacts with iron to form acorrosion product that ultimately transfers electrons to oxygen. Consistent with that model, most reportedcases of SRB induced corrosion of carbon steel in marine waters are in environments with some dissolvedoxygen in the bulk medium (Hardy and Bown, 1984; Lee et al., 1993).Others(Lee et al., 2005; Syrett, 1981)have demonstrated that the most corrosive operating condition is one in which carbon steel and copper areexposed to alternating oxygenated/anaerobic seawater.

Macrofouling

Macrofouling organisms are found at all depths and in all seas. They are most numerous along the marginsof continents where they are sometimes organized in communities of up to 500 species.

A heavy encrustation of macrofouling organisms on structural steel immersed in seawater will oftendecrease the corrosion rate of the steel as long as the cover of organisms remains complete and relativelyuniform. The heavy fouling layer acts as a barrier, limiting dissolved oxygen at the metal surface. A layer ofhard-shelled organisms, such as barnacles or mussels, on steel in the splash zone (just above the high tidelevel) also shields the metal from the damaging effect of wave action. Beneficial effects on general corrosionoccur under uniform fouling layers. If coverage is incomplete, the fouling is more likely to cause initiation oflocalized corrosion by creating oxygen concentration cells. A heavy encrustation of macroorganisms alsocan have a number of undesirable physical effects on marine structures. The fouling layer will increase bothweight and hydrodynamic drag on the structure. Interference with moving parts also may occur. A scatter ofindividual barnacles on a stainless steel surface will create oxygen concentration cells. The portion of themetal surface covered by the barnacle shell is shielded from dissolved oxygen in the water and thus becomesthe anode. The result is crevice corrosion under the base of the barnacle. A similar effect can take place onaluminum and, to a lesser extent, on structural steel.

Microfoulin! and MacrofoulinaAccelerated low water corrosion (ALWC) or lowest astronomical tide (LAT) corrosion is a

particularly aggressive form of localized corrosion that has become a high profile problem, associated withunusually high rates of metal wastage on unprotected, or inadequately protected, steel sheet pilings. Sheetpiles are used as retaining walls, wharfs, and piers. In tidal waters, pilings are exposed to a range of corrosiveenvironments, which can be classified into four zones: splash zone, tidal zone, permanent immersion, and

bed zone (Figure 5). Corrosion rates in the low water level are typically 0.1 mm/year. Average corrosionrates in the range of 0.3 to 1.2 mm/side/year are typically reported for ALWC.

ALWC phenomenon is a global phenomenon reported around the world in all climatic conditions onunprotected steel pilings in contact with saline water (i.e., seawater and brackish water) that is subject to tidalinfluences. ALWC has a distinct appearance - patches of lightly adherent, bright orange and black (ironsulfide rich) deposits over a clean, shiny and pitted steel surface (Beech et al., 1993; Cheung et al. 1994). Asthe pits deepen and become more numerous, they overlap, producing a dishing effect in the metal surface,which ultimately develops into a hole. Corrosion products contain magnetite, iron sulfides, and green rust (anunstable iron oxy-hydroxide sulfate complex). ALWC also produces a specific pattern of damage on steelsheet piling. In U-shaped piles the corrosion occurs on the outpans, while in the Z-shaped sheet piles ALWCoccurs in the web areas or comers. The pattem of damage is similar for particular pile geometries,irrespective of the geographic location of the installation.

The detailed mechanism of ALWC in marine/estuarine environments continues to be a matter ofsome debate but several research groups have concluded that it is a form of MIC. Gehrke and Sand (2003)completed a three-year study of pilings in German marine harbors with and without corrosion. Theyconcluded that the ALWC was due to the combination of SRB and thiobacilli in the fouling layers on thepilings. The organisms occurred together, separated by the oxygen gradient in the biofilm. At low tide thebiofouling layer was oxygenated whereas at high tide, anaerobic areas developed. The sulfides produced bythe SRB in the anaerobic regions and sulfuric acid resulting from the thiobacilli in the aerobic regionscombined to produce an extremely corrosive environment (Figure 6).

Conclusion

Despite the predictability of ennoblement and sulfide derivatization for passive and non-passive alloys,respectively, exposed in marine waters, their influence on corrosion is determined by environmental factorsthat are not yet predictable.

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Bibliography

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International Corrosion Congress. 3735-3746 p.Chandrasekaran, P., Dexter, S.C. 1994. Thermodynamic and kinetic factors influenced by biofilm chemistry prior topassivity breakdown. CORROSION/94; NACE International; Houston, TX. Paper no. 482.Cheung, S., Walsh, F.C., Campbell, S.A., Chao, W.T., Beech, I.B. 1994. Microbial contributions to the marinecorrosion of steel piling. International Biodeterioration & Biodegradation, 34: 259-274.de Mele, M.F.L, Brankevich, G., Videla H.A. 1989. Corrosion of CuNi30Fe in artificial solutions and natural seawater:influence of biofouling. British Corrosion Journal, 24: 211-218.Dexter, S.C. 1988. Laboratory solutions for studying corrosion of aluminum alloys in seawater. In: Francis. P.E.. Lee,T.S., editors. The use of synthetic environments for corrosion testing. ASTM STP 970. American Society for testingMaterials, Philadelphia. 217-234 p.Dexter, S.C., LaFontaine, J.P. 1998. Effect of natural marine biofilms on galvanic corrosion. Corrosion, 54: 851-861.Dexter. S.C., Maruthamuthu, S. 2001. Response of passive alloys with N- and P-type passive films to manganese inbiofilms. CORROSION/2001; NACE International; Houston, TX. Paper no. 01256.Dickinson, W., Caccavo, F., Lewandowski. Z. 1996. The ennoblement of stainless steel by manganic oxide biofouling.Corros. Sci., 38: 1407-1422.

Gehrke, T., Sand, W. 2003. Interactions between microorganisms and physicochemical factors cause MIC of steelpilings in harbors (ALWC). CORROSION/2003; NACE International; Houston. TX. Paper no. 03557.Gubner, R., Beech, I.B. 1999. Statistical assessment of the risk of biocorrosion in tidal waters. CORROSION/1999:NACE International; Houston, TX. Paper no. 184.Hamilton, W.A. 2003. Microbially influenced corrosion as a model system for the study of metal microbe interactions:a unifying electron transfer hypothesis. Biofouling, 19: 65-76 p.Hardy, J.A.. Bown, J.L. 1984. The corrosion of mild steel by biogenic sulfide films exposed to air. Corrosion. 40: 650p.Ito, K., Matsuhashi, R., Kato, T., Miki, 0., Kihira, H., Wantanabe. K., Baker. P. 2002. Potential ennoblement ofstainless steels by marine biofilms and microbial consortia analysis. CORROSION/2002, NACE International;Houston, TX. Paper no.02452.Johnson, K., Moulin, J., Karius, R., Resiak, B.. Confente, M., Chao, W.1997. Low water corrosion on steel piles inmarine waters. ECSC Final Report EUR 17868, 1997 (ISSN 1018-5593).

Johnson, R., Bardal, E. 1985. Cathodic properties of different stainless steels in natural seawater. Corrosion, 41: 296.LaQue, F. L. 1975. Marine corrosion - causes and prevention. John Wiley & Sons, New York.Lee, J.S., Ray, R.I., Little B.J. 2007.Comparison of Key West and Persian Gulf seawaters. CORROSION/2007; NACEInternational; Houston, TX. Paper no. 07518Lee, J.S., Ray, R.I., Little, B.J., Lemieux, E.J. 2006. An evaluation of ballast tank corrosion in hypoxic seawater.CORROSION / 2006, NACE International; Houston, TX. Paper no.06300.Lee, W.C., Lewandowski, Z., Okabe, S., Characklis, W.G., Avci, R. 1993. Biofouling, 7:197 p.Lloyd, B. 1937. Bacteria stored in seawater. J Roy Tech Coil, Glasgow 4. 173 p.

Martin, F.J., Dexter, S.C., Strom, M., Lemieux, E.J. 2007. Relations between seawater ennoblement selectivity andpassive film semiconductivity on Ni-Cr-Mo alloys. CORROSION/2007; NACE International; Houston, TX. Paper no.

07255.McNeil, M.B., Odom, A.L. 1994. Thermodynamic predictions of microbiologically influenced corrosion by sulfate-reducing bacteria. In: Kearns, J.R., Little, B.J., editors. Microbiologically influenced corrosion testing STP 1232.American Society for Testing Materials, Fredericksburg, VA. 173-179 p.

Motoda, S., Suzuki, Y., Shinohara, T., Tsujikawa, S. 1990. The effect of marine fbuling on the ennoblement ofelectrode potential for stainless steels. Corrosion Science, 31: 515-520.

Syrett, B.C. 1981. The mechanism of accelerated corrosion of copper-nickel alloys in sulphide-polluted seawater.Corros. Sci., 21: 187-209 p.Zhang, H-J, Dexter, S.C. 1995. Effect of biofllms on crevice corrosion of stainless steels in coastal seawater. Corrosion.51: 56-66.Zobell, C., Anderson, D.Q. 1936. Effect of volume on bacterial activity. Biol Bull, 71: 324-342 p.